Vehicles such as electric vehicles (EV) and hybrid vehicles (HV) generally obtain the driving force from electric energy by using an inverter to convert the direct-current (DC) power supplied from a high voltage battery into a three-phase alternating-current (AC) power, causing a three-phase AC motor to be rotated. During the deceleration of the vehicle, the regenerative energy obtained by regeneration of the three-phase AC motor is stored in the battery, causing the vehicle to run without wasting the energy.
In the above-described hybrid vehicle or electric vehicle, the inverter is provided with six switching elements (for example, IGBT (Isolated Gate Bipolar Transistor)) having three arms bridge-connected, and the AC power for driving the three-phase AC motor is obtained from the DC power input by the switching operation of each IGBT.
At the time of the switching operation of the IGBT, a surge voltage is generated when the IGBT is switched from on to off. This surge voltage is superimposed on the input voltage of the inverter and applied between the collector and the emitter of the IGBT in the off-state. Accordingly, in order to prevent breakdown of the IGBT, the sum of the input voltage of the inverter and the surge voltage should be limited to a value of not more than the element withstand voltage of the IGBT. In order to avoid that the sum exceeds the limitation, the input resistance is inserted in series into the gate of the IGBT, and thus, the rising and falling waveforms of the signal voltage applied to the gate are caused to be gradual by this input resistance and the parasitic capacitance between the gate and the emitter of the IGBT, with the result that the switching speed is decelerated.
However, reduction in the switching speed leads to not only a decrease in the surge voltage but also an increase in the switching loss, which produces a problem of deterioration in fuel efficiency of the vehicle. Specifically, the element withstand voltage of the IGBT has a temperature dependence that it decreases with a decrease in the element temperature. Thus, in the low temperature environment, the allowable range of the surge voltage is strictly limited and the switching loss is difficult to be suppressed.
Accordingly, Japanese Patent Laying-Open No. 2001-169407 discloses a control device for an electric vehicle provided with input resistance setting means for setting a resistance value of the input resistance of a plurality of power elements in the inverter in accordance with the driving state of the vehicle.
According to the disclosure, the input resistance setting means sets a resistance value depending on any of the battery temperature, the temperature of the power element and the power consumption of the power element. Specifically, in the region where the temperature of the power element is relatively high, as the allowable surge withstand voltage of the power element is high, the decreased input resistance value causes an increase in the switching speed, and the surge voltage is permitted to be generated to the level of the generated surge voltage limit, which allows a decrease in the switching loss. On the other hand, in the relatively low temperature region, as the allowable surge withstand voltage is low, the increased input resistance value causes the generated surge voltage to be lowered below the allowable surge withstand voltage, which allows the power element to be operated with stability.
According to the control device for the electric vehicle disclosed in Japanese Patent Laying-Open No. 2001-169407 described above, however, in the region where the power element is relatively low in temperature, while the increased input resistance value of the power element causes suppression of the surge voltage generation and allows prevention of breakdown of the power element, there is still a problem that the decrease in the switching speed results in an increase in the switching loss.
It is contemplated that the hybrid vehicle or the electric vehicle is configured such that the DC voltage from the DC power supply is boosted by the boost converter to supply the boosted DC voltage to the inverter (for example, see Japanese Patent Laying-Open Nos. 2005-198406, 2005-354763 and 2004-166341).
In such a configuration, a capacitor for smoothing the DC voltage from the boost converter is provided between the output side of the boost converter and the input side of the inverter. As the smoothing capacitor, an electrolytic capacitor, a film capacitor and the like are applied, among which an aluminum electrolytic capacitor is widely used because it allows a decrease in size and an increase in capacity.
However, in the case of the aluminum electrolytic capacitor, the real part of the impedance, that is, a so-called equivalent series resistance (ESR) has temperature characteristics that it increases with a decrease in temperature. Accordingly, in the case where the aluminum electrolytic capacitor is provided on the input side of the inverter, in the low temperature region, the voltage generated in the ESR during charging and discharging of the capacitor is increased and the increased voltage is to be superimposed on the input voltage of the inverter as voltage variation. Consequently, in the low temperature region, in order to implement both of the safe operation of the IGBT and the decrease in the switching loss, it is necessary to further consider the voltage variation due to the capacitor in addition to the surge voltage described above.
Thus, the present invention is directed to solve the above-described problems and an object of the present invention is to provide a load drive device that is capable of reliably reducing the switching loss of the drive circuit.
Another object of the present invention is to provide a vehicle equipped with a load drive device that is capable of reliably reducing the switching loss of the drive circuit.